Chapter 4 Selection of novel cowpea genotypes derived through gamma irradiation
4.4 Discussion
The present study revealed the important roles of induced mutations in cowpea breeding. It was evident from this study that increased Gy doses above 150 Gy can be lethal for the cowpea breeding line such as Nakare, while a dose above 200 Gy is lethal for the breeding line Shindimba. Other authors have reported the negative effects of increased mutagenic doses affecting various crops’ establishment and survival for breeding (Mba et al., 2009). The present study showed the presence of clear phenotypic differences among the tested mutant lines when compared to their respective controls. Visual phenotypic differences including chlorophyll, leaf, and upright single stem, pod, and seed during the M2 to M5 generations.
Chlorophyll mutants observed were plants with yellow and striped leaves, albinos or yellow to pale leaf and stem pigmentations. Virescence mutants showed broad pale green leaf breeding line such as Nakare, while a dose above 200 Gy is lethal for the breeding line Shindimba (Figure 4.2). Other authors have reported the negative effects of increased mutagenic doses affecting various crops’ establishment and survival for breeding (Mba et al., 2009).
According to Girija and Dhanavel (2009) and Maluszynski et al. (2009), the appearance of chlorophyll defects is a good indicator of genetic action of the mutagen. Singh et al. (2013) reported that increased Gy doses provided higher frequency of chlorophyll mutants in cowpea when compared to other mutagens such as EMS. Girija and Dhanavel (2009) outlined the effectiveness and efficiency of mutagens for selection of mutants with economic traits. The authors suggested that for effective phenotypic selection the mutation treatment should not yield unintended damages including chromosomal aberrations, physiological and toxic effects, which reduce cell survival and ultimately eliminate the mutation. Despite its negative effects on the early stages of crop growth, chlorophyll mutants are important in mutation breeding programs. Tulmann Neto et al. (2011) reported that the chlorophyll mutants were used in evaluation of the genetic effects and sensitivity of various mutagens on crops. These results are in agreement with Goyal and Khan (2010) whose studies indicated that the incidence of chlorophyll mutants were higher with increased Gy doses in earlier selection generations. In the present study, mutants at the M2 were genetically diverse owing to phenotypic segregation.
The genetic diversity assessed in these mutants were tall/dwarf plant heights, early/late maturity, leaf shapes, branching habit, GH, PS, FC, SC and texture, seed weight and yield Table 4.4 - Table 4.6). Both the qualitative and quantitative parameters measured in the study were useful for selection of cowpea mutants. According to Maluszynski et al. (2009), induced genetic polymorphism among initial cells of the sporogenic layer influences the segregation
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ratio in the M2 generation. However, mutations of cells of somatic tissues are not transferred to the next generation.
Gnanamurthy et al. (2012) stipulated that easily detectable mutants’ characteristics are phenotypically visible and morphologically distinct with qualitatively inherited genetic changes.
These changes occur due to the effect of few major genes or oligogenes yielding macro mutations. In this study, some macro mutations observed were the changes in flower and SC.
Micro mutations are the result of polygenes each with minor genetic effect showing quantitative inheritance. The effect and inheritance of minor genes is detected using quantitative genetic parameters and statistical methods (Singh et al., 2006). In the current study, short plant height and one seed per pod mutants were recorded in all the breeding lines mostly at the M3 generation. Single seeded pods were also reported by Girija and Dhanavel (2009).
In the present study, other main phenotypic changes observed were increased NMB especially in mutants with spreading GH. Mutants with bushy GH had reduced number of branches per plant. These characters are indicated to be associated with some physiological properties of the plant including leaf senescence and indeterminate GH (Hall, 2004; Martins et al., 2014). It is reported that characteristics altered through mutation breeding can be combined through the conventional breeding to improve crop performance and drought adaptation (Ehlers and Hall, 1997).
The present study found that Nakare mutants had a maximum of 23 main branches per plant, while the comparative control had nine main branches (Table 4.3). According to previous studies (Singh et al., 2003, 2013), the spreading and semi-spreading cowpea types yielded less grain and more fodder when planted in closer spaced rows. The present study found that mutation treatment did not significantly affect the number of days taken to germination, hence all the breeding lines germinated 3 days after planting (Table 4.3–Table 4.6). The mutation treatment had positive effect on the number of days taken to 50% flowering whereby some of the breeding lines flowered 11 days before the control. Bira mutants subjected to irradiation of 300 Gy flowered 80 days after planting (Table 4.3). Maluszynski et al. (2009) suggested that a high dose of a mutagen should yield delayed maturity. Dhanavel et al. (2008) reported that mutagenesis resulted into variation in plant development including the number of days taken to maturity. According to Singh et al. (2003), these variations are important to the farmers and the breeders allowing choices of planting time. The breeder will have a choice from a larger breeding stock for various breeding traits and purposes.
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Significant observations made in the present study were increased PL and seed yield measured during the M3 to M5 in all the breeding lines. Goyal and Khan (2010) reported that mutations caused increased PL in some of the cowpea lines. Pod size may contribute to increased seed yield. The number of grains per pod increases with increased PL though this may be associated with reduced total biomass (Singh et al., 2003). Other major effects of the mutation observed in the present study were the range of variations in SC. A mosaic of SCs were noted including white, brown, chocolate, red, speckled, cream, and black. Dhanavel et al. (2008) reported various SCs due to mutational events. The present findings suggested that the NMB per plant, NPP, number of grains per pod, 100-seed weight and seed yield per plant reduced significantly with increased concentration of irradiation doses. These findings are in agreement to the studies of Girija and Dhanavel (2009), who reported that mutagenesis is associated with negative and positive phenotypic effects for selection.
The present study demonstrated that most characters of cowpea which are of interest to plant breeders can be altered through mutations using the gamma irradiation technique.
Furthermore, new plant attributes were created in the high yielding and well adapted local cowpea varieties. Various pests were observed on mutant cowpea during this study (Figure 4.6). Therefore, there is a need to breed for insect pest tolerance in cowpea.
Timko et al. (2007) suggested that the future of cowpea improvement programs should focus on breeding for pests and diseases resistance and other desirable traits such as early maturity, photoperiod insensitivity, suitable plant type, seed quality and yield. Overall, the present study made extensive phenotypic selections of mutants from the M2 to M5 generations and identified promising genotypes. The selected mutants’ are recommended for adaptability and stability tests across representative agro-ecologies for large-scale production or breeding in Namibia or similar environments. The novel cowpea genotypes selected through the study are valuable genetic resources for genetic enhancement and breeding.
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